1. Field of the Invention
The present invention relates to a method of making thin film transistors that are used in color liquid crystal displays and the like.
2. Description of the Related Art
In general, a semiconductor device using plural thin film transistors and the like employs a multilayered wiring structure. In particular, the active-matrix type liquid crystal display requires at least two layered wirings running in the row and column directions. Further, a large panel of a liquid crystal display requires wiring having a resistance as low as possible to reduce signal delays. In fact, in order to reduce signal delays, aluminum is used for the first conductive layer of the active matrix substrate, i.e. for lower-layer wiring and gate electrodes of thin film transistors. Further, in order to electrically insulate the second conductive layer, i.e. upper-layer wiring and source-drain electrodes, from the first conductive layer and to form a gate insulating layer, an insulating layer is deposited on the lower-layer wiring and gate electrodes. Next, after thin film transistors are formed and particular areas over the gate electrodes are processed, the second conductive layer is formed to complete an array of thin film transistors.
In a matrix array of the above thin film transistors, the greatest problem has been the fact that short-circuits occur between the first conductive layer (lower-layer wiring and gate electrodes) and the second conductive layer (upper-layer wiring and source-drain electrodes). In particular, if aluminum is used for the gate electrodes and lower-layer wiring, projections called hillock are generated in the A1 thin film during a heating process such as a dehydration process which is performed at a temperature of about 160.degree. C. during a photolithographic process for forming electrodes and wiring patterns prior to coating resist. This hillock causes short-circuits between the first and second conductive layers, when a composed array of the thin film transistors are driven, after insulators, semiconductive layers, and second conductive layers, i.e. source-drain electrodes and upper-layer wiring, are formed. It is generally observed that hillocks are responsible for the short-circuits by causing cracks in the insulating layer, damaging the uniformity of thickness of the insulating layer, and causing local concentration of electric fields.
In order to solve this problem, it has been proposed to add a small amount of Ta or Ti to aluminum forming gate electrodes. However, although they are effective against hillocks, the electric resistance of the electrodes becomes about five times as high as the pure aluminum electrodes, so that they are not preferable considering the delay of gate signals. Therefore, a method of making lower-layer wiring and gate electrodes that have the electric resistance as low as pure aluminum and do not produce hillocks has been desired.